Hypogene dissolution-precipitation processes strongly affect the petrophysical properties of carbonate rocks and fluid migration pathways in sedimentary basins. In many deep carbonate reservoirs, hypogene cavernous voids are often associated with silicified horizons. The diagenesis of silica in carbonate sequences is still a poorly-investigated research topic. Studies exploring the complexity of silica dissolution-precipitation patterns in hypogene cave analogues are therefore fundamental to unravel the diagenetic and speleogenetic processes that may affect this kind of reservoir. In this work, we investigated an exhumed and silicified Neoproterozoic carbonate sequence in Brazil hosting a 1.4 km-long cave. Quartz mineralization and silicified textures were analyzed with a multidisciplinary approach combining petrography, fluid inclusion microthermometry, silicon-oxygen stable isotope analyses and U-Th-Pb dating of monazite crystals. We found that an early silicification event caused the replacement of the dolostone layers with micro-crystalline quartz forming chert nodules. This event was likely associated with mixing fluids (ancient Neoproterozoic seawater and hydrothermal solutions sourced from the underlying Mesoproterozoic basement) at relatively low temperatures (ca. 50–100°C) and shallow depth. After the tectonic deformation produced by the Brasiliano orogeny, silica dissolution was promoted by high temperature and alkaline hydrothermal solutions rising from the quartzite basement along deep-rooted structures. Hypogene hydrothermal alteration promoted the dissolution of the cherty layers and the precipitation of chalcedony and megaquartz. Homogenization temperatures from primary fluid inclusions in megaquartz cement indicate minimum formation temperatures of 165–210°C. Similar temperature estimates (110–200°C) were obtained from the δ30Si and δ18O isotope systematics of quartz precipitated from hydrothermal solutions. The dissolved salts in the fluid inclusions were evaluated as NaCl + CaCl2 from microthermometric data combined with cryogenic Raman spectroscopy, corresponding to salinity ranging between 17 and 25 wt.%. No reliable age constraints for hydrothermal silica dissolution-precipitation phases were obtained from monazite U-Th-Pb dating. However, our results, interpreted in the regional context of the São Francisco Craton, suggest that the Cambrian tectono-thermal events could have been amongst the possible drivers for this hypogene process in the basin.
Bibliographical noteFunding Information:
This research was carried out in association with the ongoing R&D project registered as ANP 20502‐1, ‘—)—Porokarst—Processes and Properties in Fractured and Karstified Carbonate Reservoirs, sponsored by Shell Brasil under the ANP R&D levy as ‘’. Cave map data were kindly provided by (GPME). Cave sampling was performed through SISBIO permit 63178/1. We sincerely thank the Iramaia municipality (State of Bahia) and the Brazilian Federal Environmental Agency () for providing access to the cave and the special permission for collecting rock samples. We thank Bastian Joachim‐Mrosko for providing the access to the Raman Spectroscopy Laboratory at the University of Innsbruck and for his valuable help. We also thank Jürgen Konzett for the early discussions on monazite dating and Augusto Auler for the help during fieldwork. We finally thank the Editor Atle Rotevatn for handling the manuscript and the reviewers Marjan Temovski, Cathy Hollis and Alexander Klimchouk for their suggestions and comments that helped to improve our paper. This article is the NordSIMS published contribution n. 726. Processos e Propriedades em Reservartorios Carbonaticos Fraturados e Carstificados POROCARSTE 3D’ (UFRN/UNB/UFRJ/UFC/Shell Brasil/ANP Compromisso de Investimento com Pesquisa e Desenvolvimento Grupo Pierre Marin de Espeleologia Instituto Chico Mendes
© 2022 International Association of Sedimentologists and European Association of Geoscientists and Engineers and John Wiley & Sons Ltd.
- fluid flow
- hydrothermal karst
- silica dissolution
- silicified reservoirs